The three requirements for complete oxidation in an incinerator are temperature, turbulence and _____.
Answer: D
Sufficient residence time lets the reaction complete once temperature and mixing have started it. Falling short on any one of the three produces partial oxidation products that may be more harmful than the original stream.
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Comparison of three flue gas desulphurisation routes.
| Route | Removal (per cent) | By-product | Relative capital cost |
|---|---|---|---|
| Dry sorbent injection | 50 to 70 | Mixed with fly ash | Low |
| Spray dryer absorption | 80 to 90 | Dry solid | Moderate |
| Wet limestone | 95 to 98 | Saleable gypsum | High |
| Wet limestone with polishing | over 99 | Saleable gypsum | Highest |
Which route suits a small plant with no gypsum market and a modest emission limit?
Answer: A
Correct answer: Spray dryer absorption
The plant is small, so the high capital cost of a wet limestone system is hard to justify; it has no gypsum market, so the wet route’s chief economic advantage disappears; and the limit is modest, so 80 to 90 per cent removal suffices. Spray dryer absorption meets all three: moderate cost, a dry solid product handled with the fly ash, and no liquid effluent or reheat. Dry sorbent injection would probably fall short of the limit.
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Methods for volatile organic compounds by concentration.
| Concentration | Preferred method | Reason |
|---|---|---|
| Very low, large volume | Biofiltration | Combustion would need prohibitive fuel |
| Low to moderate | Adsorption | Recovery or destruction after concentration |
| Moderate to high | Catalytic incineration | Self-sustaining with modest preheat |
| Very high, recoverable | Condensation | Material has value as a liquid |
Why is incineration unsuitable for a very dilute stream?
Answer: B
Correct answer: Heating the whole volume costs more than it yields
An incinerator must raise the entire gas stream to the oxidation temperature, and in a dilute stream almost all of that gas is air carrying no calorific value at all. The fuel bill is set by the volume, while the benefit is set by the pollutant mass, so the economics collapse as the concentration falls. The pollutant would oxidise perfectly well; it is simply not worth what it costs to heat the air around it.
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Effect of operating variables on activated carbon working capacity.
| Variable | Change | Effect on capacity |
|---|---|---|
| Temperature | Increase | Decrease |
| Humidity | Increase | Decrease |
| Pollutant concentration | Increase | Increase |
| Competing organics | Increase | Decrease |
What pre-treatment does the table suggest for a warm humid stream?
Answer: C
Correct answer: Cool and dry the gas beforehand
The table shows capacity falling with both temperature and humidity, so a warm humid stream is doubly disadvantaged. Cooling raises the capacity directly and, by condensing out moisture, removes the water that would otherwise compete for the adsorption sites - though cooling alone raises the relative humidity, so a condensing cooler or a subsequent reheat is used. Heating would lower the relative humidity but reduce capacity through the temperature effect.
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Selective catalytic and non-catalytic reduction compared.
| Attribute | Catalytic | Non-catalytic |
|---|---|---|
| Removal efficiency | 80 to 90 per cent | 30 to 50 per cent |
| Operating temperature | 300 to 400 C | 850 to 1100 C |
| Catalyst required | Yes | No |
| Relative capital cost | High | Low |
Why might a plant choose the non-catalytic route despite its lower removal?
Answer: D
Correct answer: Its far lower capital cost, and no catalyst
The non-catalytic route needs only injection lances into the furnace: no reactor vessel, no catalyst to buy, house, monitor and replace every few years, and no additional pressure drop for the fans to overcome. Where the emission limit can be met with 30 to 50 per cent removal, that saving decides the matter. Its operating temperature is far higher, not lower, and its reagent consumption is greater rather than less.
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Absorption tower sizing for four separations.
| Separation | Transfer units required | Height of a transfer unit (m) | Packed height (m) |
|---|---|---|---|
| Easy, very soluble gas | 2.0 | 0.45 | 0.90 |
| Moderate | 4.0 | 0.50 | 2.00 |
| Difficult | 6.4 | 0.55 | 3.52 |
| Very difficult | 9.0 | 0.60 | 5.40 |
What does the table separate into two distinct factors?
Answer: A
Transfer units measure how hard the separation is and the height of a transfer unit measures how well the packing performs. Improving the packing shortens the tower without changing the number of units the duty demands.
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Assertion (A): Adsorption beds are switched at breakthrough rather than at saturation.
Reason (R): Continuing past breakthrough buys the remaining capacity at the price of an emission.
Select the correct answer from the options below.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason states the trade exactly. Between breakthrough and saturation the bed still has capacity, but every hour of that capacity is bought by emitting pollutant at a rising concentration. Since the whole purpose of the bed is to prevent that emission, it is switched at breakthrough, and the design capacity used is the working capacity up to that point rather than the equilibrium capacity.
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Assertion (A): Catalytic incineration is preferred where fuel cost dominates.
Reason (R): A catalyst allows oxidation at roughly four hundred degrees rather than eight hundred.
Select the correct answer from the options below.
Answer: C
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason is the mechanism. The fuel needed is what it takes to raise the whole gas stream to the operating temperature, so halving that temperature roughly halves the heat duty. Where fuel dominates the operating cost - a continuous stream of moderate concentration - catalytic incineration wins, provided the stream is free of the halogens, sulphur and metals that would poison the catalyst.
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Assertion (A): Wet limestone desulphurisation usually requires flue gas reheat.
Reason (R): The gas leaves the absorber cooled and saturated with water vapour.
Select the correct answer from the options below.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the cause. The gas leaves a wet absorber at 50 to 60 degrees and saturated, so it has little buoyancy and will condense acid in the ductwork and stack. Reheating it by 20 or 30 degrees restores plume rise and protects the downstream metal, at the cost of fuel or of a gas-to-gas heat exchanger. Avoiding this is one of the arguments for the semi-dry routes.
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Assertion (A): Ammonia slip rises as removal efficiency is pushed higher.
Reason (R): Higher removal requires a higher ammonia to nitrogen oxide molar ratio.
Select the correct answer from the options below.
Answer: A
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason explains the mechanism. Mixing between reagent and flue gas is never perfect, so pushing removal higher requires injecting more ammonia than stoichiometry demands; the fraction that finds no nitrogen oxide passes through as slip. Removal and slip therefore rise together, and the practical limit on removal is usually set by the slip the downstream equipment and the permit can tolerate.
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